Homogeneous and Heterogeneous Catalytic Phenomena
Homogeneous and Heterogeneous Catalytic Phenomena
A Review of the Second International Colloquium
At an international colloquium sponsored by the Centre National de la Recherche and held at Lyon in November 1977 the four principal lectures, and several of the supporting ones, featured the use of platinum group metals as catalysts. The main objective of this meeting was to bring together chemists working with heterogeneous catalytic reactions and others specialising in inorganic and organometallic chemistry in order that they could together consider the relations between homogeneous and heterogeneous catalytic phenomena.
Throughout the conference the theme was to explore that intermediate area of catalytic behaviour which lies between the homogeneous systems on the one hand and the heterogeneous systems on the other. There were four plenary lectures. The first, Homogeneous and Heterogeneous Catalysis—Kinetic and Mechanistic Aspects, by Professor J. Halpern, of the University of Chicago, provided a detailed analysis of Wilkinson's catalyst (Ph3P)3RhCl. The enormous complexity of catalytic hydrogenation of alkenes by this complex was explored. Halpern, in his summary, drew attention to the problem of making realistic comparisons between the truly homogeneous and truly heterogeneous, or supported homogeneous, catalysts.
The second plenary lecture—Cluster Compounds and their possible relation to Heterogeneous Catalysts—was given by Dr. B. F. G. Johnson, of the University of Cambridge, who summarised the contribution that studies of cluster complexes have made to the understanding of the chemistry occurring on macroscopic crystal surfaces. He took as his examples the cluster carbonyls of osmium for which a series of derivatives based on Os3, Os4, Os5, Os6, Os7 and Os8 units has been established. The structural relationship between small groups of metals and the bulk metal structure was examined. Bonding modes adopted by small molecules such as CO, H, alkenes and alkynes with triangular, square planar, and butterfly arrangements of metal atoms were described as possible models for the binding of such molecules to the bulk metal.
Professor C. U. Pittman, University of Alabama, took as his subject the chemistry of supported homogeneous catalysts, in particular those based on rhodium and platinum compounds. In his lecture, Catalytic Reactions over Anchored Complexes on Polymers, he developed the theme that many of the problems associated with homogeneous catalysts, such as loss of catalyst due to difficulty in retrieving precious metal salts, may be resolved by the fixation of the catalyst to a rigid support. He was able to demonstrate that variation in support design could be used to modify or regulate catalyst activity but conceded that, in general, anchored catalysts were less efficient than their strictly homogeneous counterparts.
The final plenary lecture—Oxidation Reactions in Homogeneous and Heterogeneous Phases: Oxygen Activation—by Professor R. Ugo, of the Instituto di Chimica Generale, Milan, was devoted to catalytic oxidation. In a wide-ranging lecture Professor Ugo first discussed the role of silver in the catalytic oxygenation of ethylene to ethylene epoxide by oxygen. He continued with a summary of the work by Sharpless and the study of the stoichiometric oxidation of olefins by metallo-oxygen and seleno-oxygen compounds. He reported on stoichiometric oxygenation by the oxygen complexes of the platinum metals. The role such complexes might play as catalysts in the homogeneous phase was appraised. Throughout considerable emphasis was placed on the mechanisms by which these various oxidation reactions might occur.
These lectures will be published in full in a future volume of the Journal of Catalysis. Many of the supporting lectures, some briefly mentioned below, have been published in the same journal.
In a supporting lecture Dr. M. Bartholin, L.M.O., Villeurbanne, France, described the fixation and evolution of organometallic rhodium complexes supported on modified polyphenylsiloxane and silica. Oxidative-additions with C-H fission at triosmium clusters was reported by Dr. A. J. Deeming, of University College, London, and asymmetric synthesis via polymer attached optically active platinum catalysts by Dr. J. Stille, of Colorado State University. Dr. Z. M. Michalsha, Institute of Polymers Technical University, Lodz, Poland, presented a paper on the catalytic activity of supported rhodium (I) and platinum(0) complexes in hydrosilylation reactions, and A. Guyot a paper on the influence of the texture of phosphinated polystyrene resins on the stability and catalytic activity of supported rhodium complexes. Papers by G. Sbrana and Dr. R. H. Grubbs considered the catalytic activity of ruthenium (II) and rhodium (I) catalysts bound to polycarbonylate and poly-B-diketonate matrices, and hybrid catalystmetathesis catalysts attached to polystyrene copolymer, respectively.